1. Field of the Invention
The present invention pertains to receiver coils and, in particular, to receiver coils used in conjunction with a Nuclear Magnetic Resonance Imaging instrument.
2. Description of the Prior Art
Nuclear Magnetic Resonance Imaging (NMRI) instruments are used today in a wide variety of analytical and diagnostic applications. An object to be analyzed in an NMRI instrument is positioned within a strong magnetic field. A transmitter, in the form of a coil, is connected to a source of radio frequency (RF) energy and, in response thereto, the transmitter coil irradiates the object with an alternating magnetic field. Constituents of the object absorb energy from the alternating magnetic field and, as a result, certain energy levels of the constituents are excited. That is, selected nuclei are energized from their equilibrium states to higher energy states.
Subsequently, the constituents return to their initial, lower energy level by radiating some of the absorbed energy. This radiated energy is detected by a receiver coil. Finally, standard, well-known Nuclear Magnetic Resonance (NMR) techniques ar used to analyze the signals produced by the receiver coils. Such standard, well-known NMR techniques are disclosed in a number of references, for example: an article entitled "Medical Imaging by NMR" by P. Mansfield and A. A. Maudsley, British Journal of Radiology, Vol. 50, pp. 188-194 (1977); an article entitled "Image Formation by Nuclear Magnetic Resonance: The Sensitive-Point Method" by W. S. Hinshaw, Journal of Applied Physics, Vol. 47, No. 8, Aug. 1976; U.S. Pat. No. 4,015,196 issued to Moore et al.; and U.S. Pat. No. 3,932,305 issued to Abe et al.
The receiver coil in a typical NMRI instrument that is designed for providing images of humans of medical purposes is necessarily large in order to accommodate an entire human body. For example, see U.S. Pat. Nos. 4,354,499 and 4,411,270 both of which were issued to Damadian. However, such large receiver coils pose a problem when a smaller object such as, for example, a human neck, a joint, a limb, and so forth, is analyzed. Such small objects fill only a portion of the volume of larger receiver coils, thus resulting in a diminished signal-to-noise ratio.
Thus, there is a need in the art for receiver coils used in conjunction with NMRI instruments for analyzing relatively small objects such as the human head which provide substantially uniform sensitivity and high signal-to-noise ratios.